> pn] I've noticed when FTPing large files between two machines on the same
> subnet/hub, the collision light alternates between flickering and almost
> solid-on. Is this normal and of any real concern? Does it have anything
to
> do with hub "quality?"
This has nothing to do with hub "quality", and everything to do with the
ethernet specification. When running a hub, you will get collisions. It's
just how ethernet works. Since the hubs usually stretch the LED on time for
colisions & data (ie any data or colision turns the LED on for about .2
seconds), you will frequently see the LED's stay on solid, even though
you're obviously not transmitting data or incurring collisions 100% of the
time.
> > Are they big files that should tax the network? If so, you're probably
> > maxing out the network in which case it's "normal" and not a reason for
> > concern. I've heard people target 5% collisions. You can check each
> > machine's interface with ifconfig (or, I guess newer distro's use ip):
I
> > believe you divide the collisions by the bytes out. If the error
> > rate seems
> > high then maybe you have some trouble to worry about.
> >
> > As I understand it, you should be able to get about half the bandwidth.
A
> > 100 Mb connection will transfer 50*1000*1024/8 bytes per second (half
the
> > 100 times a million bits divided by 8 bits per byte). Are you
> > getting these
> > kind of throughputs?
>
> pn] Well, I just ftp'ed on again to check. It transferred a 639,453,184
> byte file in 132.83 seconds for 4814.11Kbytes/sec. Isn't this only about
> 38.5Mb/sec? This is with no other significant activity on the network.
Not
> that's it's bad (overall), but I'm just wondering if it is normal or if it
> indicates a problem somewhere.
This looks pretty normal to me.
You generally won't saturate an ethernet connection with a single
machine-machine connection (think of the problems that would result if you
did). I'd bet if you run multiple FTP connections between your two boxes at
the same time, you'll wind up with more aggregate bandwidth (ie each
connection will be a bit slower, but the total of all of them will be faster
than the 38.5 Mb/sec you see with one). You'll also see more bandwidth used
if you add more than two machines to the network (up to a point, then
excessive collisions start to reduce the effective bandwidth, and it's time
to buy a switch).
If you really need to, you can probably tweak some TCP/IP parameters and get
a single FTP connection to come much closer to saturating your ethernet link
(somewhere around 70-80 Mb/sec, or roughly 2x what you're measuring),
assuming your CPU's, NIC's, and other infrastructure are not the bottleneck.
Doing this, however, could very well cause some other applications (like
telnet/ssh, which send small packets and want low latency) to perform worse.
Charles Steinkuehler
http://lrp.steinkuehler.net
http://c0wz.steinkuehler.net (lrp.c0wz.com mirror)
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